This invention relates to tamperproof induction watthour meters and more particularly to such watthour meters including an improved protected testing arrangement for preventing unathorized tampering of the meter from outside of the meter enclosure.
It is known to provide self-contained watthour meters of the detachable or socket-type so as to minimize tampering of the meter movement or associated metering circuits to alter or decrease accurate electric power billing registration of the meter. In copending application Ser. No. 485,675, filed by Finnen et al., July 3, 1974, concurrently with this application, a permanently locked watthour meter enclosure is disclosed and claimed for preventing access to the watthour meter movement by separating the meter cover from the meter base assembly. Such meters are often referred to as secure or tamperproof meters since they include additional anti-tamper design features from more conventionally provided watthour meter design features.
Even in watthour meters which are permanently locked, as disclosed in the aforementioned copending application, it is possible upon removal of the watthour meter from a meter socket to open a voltage winding disconnect link which is universally provided in commercially available watthour meters at the rear of the meter base assembly. For normal meter operation, the disconnect or potential link is positioned to externally close two auxiliary terminals to interconnect one end of the meter voltage winding and a terminal blade. If the meter is removed from the utility customer's socket and the disconnect link is moved to an open position and the meter is replaced in the meter socket, the customer's load is connected through the meter to the utility company's power lines, but the watthour meter movement will not operate since the voltage winding is deenergized.
One of the two auxiliary terminals associated with a disconnect link forms a test contact having a standardized predetermined external location on the meter base assembly. The link includes a segment of conductive material to provide a conductive path between the second auxiliary terminal and the auxiliary terminal having the test contact. The test contact terminal is connected internally directly to one terminal end of the voltage winding and the second auxiliary terminal is connected internally to one of the terminal blades. In Electrical Metermen's Handbook, Seventh Edition, published 1965 by Edison Electric Institute, 750 Third Avenue, New York, N.Y. 10017 in Chapters 15 and 19, watthour meter testing circuits are described including the so-called open-link method. This open-link method is one of the most common commercial methods of testing watthour meters. In the open-link method, the watthour meter metering circuit is tested by short-circuiting the meter current windings. The short circuit is accomplished by placing a wire jumper across the two bottom terminal blades and disconnecting one terminal end of the voltage winding from one of the upper terminal blades. Opening the disconnect link on the outside of the meter base assembly isolates the voltage winding from the short circuit test connection and the external test contact provides connection of the voltage winding to the test circuit. The open-link meter test circuits are included in test facilities including test tables or boards having meter test sockets. These meter test sockets usually include a spring biased test terminal positioned for contacting the test contact on the meter base assembly. Therefore, it is a standard practice for meter manufacturers to provide such test contacts at a standardized location on the meter base. The test contacts are then adapted for universal use in the standard meter test sockets of commercial watthour meter test facilities typically used by meter manufacturers and electric utility meter test departments.
In the U.S. Pat. No. 2,701,859 issued Feb. 8, 1955 to Coleman, a watthour meter is disclosed including a test circuit which includes test terminals that are wholly located within the meter enclosure. While the cover must be removed to perform testing, as in the instant invention, there are no externally exposed test contacts disclosed connected with the watthour metering circuit.
Accordingly, it is desirable to provide a secure watthour meter including a testing arrangement for performing open-link test circuit methods at an external test contact while preventing tampering of the meter externally of the meter enclosure to cause deenergization of the meter voltage winding circuit after it has been altered and the meter is replaced in service to connect a customer's load to an electric utility company's power lines.